1. Field of the Invention
The present invention relates to a voltage reference generator, and more particularly, to a low voltage bandgap reference circuit.
2. Description of the Prior Art
The voltage reference generator is an essential design block generally needed in analog and mixed circuits. It typically uses a bandgap reference circuit to generate a reference voltage that is relatively insensitive to the temperature and the supply voltage. The reference voltage output of the bandgap reference circuit according to the prior art is about 1.2V that is roughly equal to silicon bandgap energy measured at 0K in electron volts. Thus, the required supply voltage is at least 1.4V or higher.
The base-emitter voltage of the bipolar junction transistor (BJT) and the voltage difference between the base and the emitter of two BJTs are main factors determining the reference voltage. The base-emitter voltage has a negative temperature coefficient; that is, the base-emitter voltage decreases as the temperature increases. On the other hand, the voltage difference between the base and the emitter has a positive temperature coefficient; that is, the voltage difference between the base and the emitter increases as the temperature increases. To prevent the reference voltage varying as the temperature, the voltage difference between the base and the emitter is adjusted and added to the base-emitter voltage.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a bandgap reference circuit 10 according to the prior art. The bandgap reference circuit 10 includes an operation amplifier OP0, two transistors M0 and M1, and two resistors R0 and R1. In complementary metal oxide semiconductor (CMOS) process, the parasitic diodes can be formed with the vertical junction p+/n-well/p-sub of the bipolar transistor having the collector and the base connected to the ground. The base-emitter voltage of a forward active operation diode can be expressed as:Vbe=Vt*ln(Ic/Is)Vt=kT/q 
Where Ic is the collector current, Is is the saturation current, k is Boltzmann constant, T is temperature, q is electron charges, and Vt is the thermal voltage. Vt is about 26 mV at room temperature (˜300K).
The voltage across the resistor R0 is the voltage difference between the voltage Vbe1 and Vbe0, which can be expressed as:ΔVbe=Vbe1−Vbe0=Vt*ln(n)
Where Vbe1 is the base-emitter voltage of the diode Q1, Vbe is the base-emitter voltage of the diode Q0. When the diode Q1 is n times the size of the diode Q2, the current through the resistor R1 is the same as that through the resistor R0. The output reference voltage can be expressed as:
  Vref  =                    Vbe        ⁢                                  ⁢        1            +              R        ⁢                                  ⁢        1        *                              Vt            *                          ln              ⁡                              (                n                )                                                          R            ⁢                                                  ⁢            0                                =                  Vbe        ⁢                                  ⁢        1            +              Vt        *        M            
The base-emitter voltage typically has a value of 0.6V and a negative temperature coefficient of −2 mV/K (complementary to absolute temperature, CTAT). The thermal voltage has a positive temperature coefficient of +0.085 mV/K (proportional to absolute temperature, PTAT). Thus, the output reference voltage can be insensitive to the temperature. When M=23, the reference voltage is about 0.6V+23*26 mV˜1.2V.
However, the bandgap reference circuit 10 according to the prior art in FIG. 1 cannot be applied in the low supply voltage applications or be implemented by the deep submicron CMOS device where the power supply VDD is less than 1.2V. Thus, the prior art provides a low voltage bandgap reference circuit. Please refer to FIG. 2. FIG. 2 is a schematic diagram of a low voltage bandgap reference circuit 20 according to the prior art. The bandgap reference circuit 20 includes an operation amplifier OP0, three transistors M0, M1 and M2, four resistors R0, R1a, R1b and R2, and two diodes Q0 and Q1. The output reference voltage can be expressed as:
                    Vref        =                ⁢                  R          ⁢                                          ⁢          2          *                      (                          ICTAT              +              IPTAT                        )                                                  =                ⁢                  R          ⁢                                          ⁢          2          *                      (                                                            Vbe                  ⁢                                                                          ⁢                  1                                                  R                  ⁢                                                                          ⁢                  1                  ⁢                  a                                            +                                                V                  ⁢                                                                          ⁢                  t                  *                                      ln                    ⁡                                          (                      n                      )                                                                                        R                  ⁢                                                                          ⁢                  0                                                      )                                                  =                ⁢                                            R              ⁢                                                          ⁢              2                                      R              ⁢                                                          ⁢              1              ⁢              a                                *                      (                                          Vbe                ⁢                                                                  ⁢                1                            +                              R                ⁢                                                                  ⁢                1                ⁢                a                *                                                      Vt                    *                                          ln                      ⁡                                              (                        n                        )                                                                                                  R                    ⁢                                                                                  ⁢                    0                                                                        )                                                  ∼                ⁢                                            R              ⁢                                                          ⁢              2                                      R              ⁢                                                          ⁢              1                                *          1.2          ⁢                                          ⁢          V                    
In conclusion, the bandgap reference circuit can provide a stable output voltage insensitive to the temperature and the supply voltage. The output reference voltage of the bandgap reference circuit according to the prior art is about 1.2V, so the required supply voltage VDD is at least 1.4V or higher. However, in the deep submicron CMOS device where the power supply VDD is less than 1.2V, the low voltage bandgap reference circuit is used.